Many electronic devices have operational speed requirements that may result in heat dissipation levels that can not be conducted away from the electronic device fast enough to prevent device failure or reduction in operating lifetime. It is known to use heat sinks, heat pipes and integrated heat spreaders on high powered electronic devices such as microprocessors, to improve the rate of thermal conduction away from the device.
The maximum rate at which heat may be conducted away from electronic devices depends in part upon the thermal conductivity of the various materials situated between the heat generator, for example the junction of an IC, and the air or liquid material that will carry the heat away from the electronic device. In an IC these materials may include the die attach material connecting the IC to the package, the package material itself (for example multiple ceramic layers in a hermetic package), the interface material between the package surface and the heat sink (for example a silver filled epoxy or a solder), and the metal of the heat sink itself. These materials must be designed for multiple requirements, only one of which is thermal conductivity.
In general, metallic compounds tend to have relatively high thermal conductivity as compared to organic compounds such as epoxy, and may be used to make thermally conductive joints between a heat-producer and a heat-dissipater. Examples include using a solder to physically and thermally attach an IC to the inside cavity of a circuit package, or to physically and thermally attach a heat sink to the outer surface of a circuit package. One issue with the use of metals such as solders in making physical attachments may be the thermal coefficient of expansion (“TCE”) difference between the two devices, such as an IC package, and an aluminum heat sink. The TCE mismatch may occur during heating and cooling phases of device operation. This TCE difference may cause stress on the attachment area of the heat sink to the IC package, and may cause device failure or reduced operational lifetime. As a result of the TCE mismatch issue, it is known to use a local point of physical attachment, such as a screw or metal clip, to provide the majority of physical attachment between a heat sink and an IC package, and to provide a flexible thermal interface material (“TIM”) between the heat sink and the IC package to improve the thermal attachment conductivity.
Another issue with the use of metals such as solder in making either physical or thermal attachments may be the tendency of metals to oxidize and form thermally and electrically insulating layers. These insulating oxide layers may be an issue particularly at the surface of an IC package and at the surface of the heat sink. Such oxidation may result in crack formation in the body of the solder, and separation of the metal attachment from the IC package surface, resulting in device failure. Thus there is a need in the art for a thermally conductive thermal interface material that reduces the stress due to TCE mismatch and reduces the oxidation of the IC package to attachment interface.